The need to wash blanket cylinders during an offset web printing process is well established. As printed copy is produced, debris from ink, paper and fountain solution cause the blanket cylinder to become contaminated. After a period of time, the blanket cylinders must be washed with either solvent or a combination of solvent and water to remove this debris. If washing does not occur, print quality will be reduced and the probability that a web will break increases. Several methods are known in the art to wash blanket cylinders.
The hand wash method utilizes manual application of wash fluid by the press crew when the press is stopped. In this method, hand towels are soaked and the blankets physically scrubbed clean of debris. Manual washing of the blanket while the press is operating is also utilized. A member of the press crew will clean a specific area of a blanket that is causing print quality to deteriorate. While this technique extends productivity, it is very unsafe because of the high peripheral speed of the blanket cylinder.
It is far safer, more productive and predictable to use automatic blanket cleaners to clean the blanket cylinder at regular intervals while printing. Several automatic methods have been employed to accomplish this.
Cloth type systems, described in U.S. Pat. Nos. 4,344,361 and 4,757,763, utilize a disposable cloth that passes over an inflatable pressure pad. The cloth is wetted with wash fluid pressed against the blanket and the cloth is advanced in increments to affect cleaning.
Brush type systems utilize a rotating brush that is pressed against the blanket. The brush may be wetted with solvent prior to coming in contact with the blanket. Debris that is removed from the blanket to the brush is gathered in a used solvent collection system. Typically the solvent undergoes separation and treatment to remove the debris and collect the solvent.
Spray type systems utilize a spray bar positioned near the blankets. The spray bar sprays wash fluid onto the blanket cylinders. Previously known spray bars, as in U.S. Pat. No. 4,686,902, premix solvent and water in a tank to form an effective emulsified solution. This is undesirable because once the wash solvent is mixed, the ratio of solvent to water in the solution cannot be changed unless the system is drained and flushed and a new ratio established. Additionally, the mixture must be continually circulated or agitated to maintain the emulsification. The present invention avoids the need for mixing water and solvent in a tank or premixer and avoids the need for recirculation or agitation of the wash solvent, thus allowing continual change in the emulsified solution as desired without requiring that the system be drained and flushed.
In other known spray systems, for example German Patent No. 2,826,135, the amount of wash fluid sprayed by the spray bar is controlled and limited to the volume of the internal cavity of the spray bar. In particular, the system works in cycles, where one cycle consists of completely filling the internal cavity of the spray bar with wash fluid followed by activating the air for spraying. The present invention avoids this cyclic action by providing a means for the wash fluid to be continuously ejected from the spray bar as it enters the spray bar. The present invention also provides for individually settable water and solvent amounts which are not limited to the volume of the internal cavity of the spray bar.
Other problems encountered in conventional cleaning systems include the risk of dryer damage or even explosion caused by the ignition of excessive solvents. Typically when the press is shut down for maintenance or repair the ink roller train becomes tacky from ink residue and debris. If the press is restarted in this condition there is a high risk of web breakage. In order to avoid this breakage, the blanket is prewet manually by a pressman applying solvent to the blanket or adding oil to the roller train to soften the tacky ink. This practice is both hazardous and wasteful because uncontrolled amounts of solvent entering the dryer can result in explosion. Also, the added oil has a deleterious effect on the ink color resulting in paper waste while the oil is dissipated. The present invention avoids this problem by providing a means to automatically prewet the blanket, reducing the risk of explosion from uncontrolled solvent application.